This chapter is about getting images into GIMP. It explains how to
create new images, how to load images from files, how to scan them and
how to make screenshots.

But first we want to introduce you to the general structure of images
in GIMP.

1. Image Types

It is tempting to think of an image as
something that corresponds with a single display window, or to a
single file such as a JPEG
file. In reality, however, a GIMP image is a
a complicated structure,
containing a stack of layers plus several other types of objects:
a selection mask, a set of channels, a set of paths, an "undo"
history, etc. In this section we take a detailed
look at the components of a GIMP image,
and the things that you can do with them.

The most basic property of an image is its
mode. There are three possible modes: RGB,
grayscale, and indexed. RGB stands
for Red-Green-Blue, and indicates that each point in the image
is represented by a “red” level, a “green”
level, and a “blue”
level; representing a full-color image.
Each color channel has 256 possible intensity
levels. More details in
Color Models

In a grayscale image, each point is represented by a brightness
value, ranging from 0 (black) to 255 (white), with intermediate
values representing different levels of gray.

Figure 5.1. Components of the RGB and CMY Color Model

In the RGB Color Model, mixing Red, Green and Blue gives White,
which is what happens on your screen.

In the CMY(K) color model, mixing Cyan, Magenta and Yellow gives
Black, which is what happens when you print on a white paper. The
printer will actually use the black cartridge for economical
reasons and better color rendering.

Conceptually, the difference between a grayscale image and an RGB
image is the number of “color channels”: a grayscale image
has one; an RGB image has three. An RGB image can be thought of as
three superimposed grayscale images, one colored red, one green,
and one blue.

Actually, both RGB and grayscale images have one additional
color channel called the alpha channel, which
represents opacity. When the alpha value at a given location
in a given layer is zero, the layer is completely transparent
(you can see through it),
and the color at that location is determined by what lies
underneath. When alpha is maximal (255), the layer is opaque
(you cannot see through it), and the
color is determined by the color of the layer. Intermediate
alpha values correspond to varying degrees of transparency / opacity:
the
color at the location is a proportional mixture of color from the
layer and color from underneath.

Figure 5.2. Example of an image in RGB and Grayscale mode

An image in RGB mode, with the channels corresponding to Red,
Green and Blue.

An image in Grayscale mode, with the channel corresponding to
Luminosity.

In GIMP, every color channel, including the alpha
channel, has a range
of possible values from 0 to 255; in computing terminology, a depth of 8
bits. Some digital cameras can produce image files with a depth of 16
bits per color channel. GIMP cannot load such a file
without losing
resolution. In most cases the effects are too subtle to be detected by
the human eye, but in some cases, mainly where there are large areas
with slowly varying color gradients, the difference may be perceptible.

Figure 5.3. Example of an image with alpha channel

Red channel

Green channel

Blue channel

The Alpha channel shows the image area which is transparent.

A color image in RGB mode with an Alpha channel.

The third type, indexed
images, is a bit more complicated to understand. In an indexed image,
only a limited set of discrete colors are used, usually 256 or less.
These colors form the “colormap” of the image, and each
point in the image is assigned a color from the colormap. Indexed images
have the advantage that they can be represented inside a computer in a
way which consumes relatively little memory, and back in the dark ages
(say, ten years ago), they were very commonly used. As time goes on, they
are used less and less, but they are still important enough to be worth
supporting in GIMP. (Also, there are a few important
kinds of image
manipulation that are easier to implement with indexed images than with
continuous-color RGB images.)

Some very commonly used types of files (including
GIF and
PNG) produce
indexed images when they are opened in GIMP. Many of
GIMP's
tools don't work very well on indexed images–and many filters
don't work at all–because of the limited number of colors
available. Because of this, it is usually best to convert an
image to RGB mode before working on it. If necessary, you can
convert it back to indexed mode when you are ready to save it

GIMP makes it easy to convert from one image type to
another,
using the Mode command in
the Image menu. Some types of conversions, of course (RGB to
grayscale or indexed, for example) lose information that cannot
be regained by converting back in the other direction.

Note

If you are trying to use a filter on an image, and it appears grayed out
in the menu, usually the cause is that the image (or, more specifically,
the layer) you are working on is the wrong type. Many filters can't be
used on indexed images. Some can be used only on RGB images, or only on
grayscale images. Some also require the presence or absence of an alpha
channel. Usually the fix is to convert the image to a different type,
most commonly RGB.